In a sports racket which uses strings such as tennis, squash etc. plastic grommet is used to guide a string through the interior of the frame through a string hole and exitting to the string network, so as to prevent the string from cutting into the stringframe contact surface which occurs when the frame is made of softer material such as graphite or composites, or to prevent the sharp edge of the hole to cut the string when the frame is made of metal. For high quality rackets, the grommets are molded in the form of a long band called in the trade as a grommet strip. The grommet strip at the head of the frame is occasionally designed to spread out along its width to cover the side of the frame, called wings, at the head portion so as to protect the surface of the frame head when the racket accidently hits the ground during play. There usually is a recess made in the frame of the head portion of the racket to receive the central part of the grommet strip to make the string less exposed so that the string will not be damaged when the head frame hits the ground accidently during play. Such grommet strip at the head of the frame of a racket is often called bumper guard which combines the function to guard the string as well as the grommet strip. In the specification here, bumper guard is the grommet strip used at the head portion which has wings spreading laterally to guard the frame; whereas a grommet strip is a narrow strip which has multiple grommets but does not protect the frame's surface.
When the frame vibrates at high frequency upon the impact of a ball which lasts only for a few thousandth of a second, the impact sets the network and the frame in motion with a sudden acceleration of which the attenuation of amplitude is slow because the hysteresis loss due to the internal macroscopic movement of the material under stress is small. Among the vibrating parts of the frame, vibration in the head portion of the frame is more severe and attenuation is slow because the head mass, incluiding the nearby strings and bumper guard, behaves as an unsupported overhanging cantilever, far way from the center of the racket and the handle. Prior art has seen a variety of plane forms of frame head design for the purpose of creating different bending rigidity and different vibration mode shapes to reduce vibration, but mostly they are not effective, in spite of racket manufacturers' claims to the contrary.
The invention recognizes an essential fact that for effective vibration design the deformation of the head frame caused by the vibration has to be constrained structurally and the vibratory energy is to be consumed more effectively than hitherto demonstrated in prior art, since the space available and weight consideration in the head frame impose a severely constrained design environment. By fitting a thin piece of rubber sheet under the bumper guard, between the bumper guard and the frame, not fastened with either surface, as was being done occastionally in prior art, is almost worthless to supress vibration at the head. It is because rubber is a poor damping material, it consumes no energy when sandwiched between the bumper guard and the frame.
The invention suggests that the damping layer used in the head frame has to be very efficient in damping. For example, in the case of a tennis racket, a designer can only afford to add at most 10 gm. in extra weight for damping purpose, for a 40 cm head frame length corresponding to the length of a bumper guard. Assuming a frame width of about 20 mm, and a material density of about 1.3, the thickness of the damping layer sandwiched in between the bumper guard and the frame can not be thicker than 1.0 mm. If width is reduced to 18 mm and weight allowance is increased to 15 gm, the thickness of the extra damping layer may be increased to 1.6 mm. This may be the upper limit to add a damping layer beneath the bumper guard in the head frame. With that little thickness to work on, the layer has to be very efficient in damping and ordinary elastomers or similar materials just wont't do.
Furthermore, how the vibration is to be damped, by shear or by extensional deformation is also important for this type of application. The damping layer should be tightly attached to the moving surface of the vibrating frame. Since the available thickness is so small, less than 1.5 mm thick, we should choose a damping material which restricts vibration principally by shear-damping which resists shear deformation across the thickness, not along the length. That is, the material resists shear movement much more effectively than it resists stretching. A characteristic of such material is that is difficult to be cut by a knife or by a pair of scissors. Such instruments cut a sheet by forcing an abrupt discontinuity in shear deformation. For scissors, it forces one edge up and the same edge down to achieve a breakage. So, for such material, a small amplitude movement in the direction of the vibrating surface of the frame will be picked up and resisted by the damping layer. Such shear resistance will be greatly enhanced if the other surface of the damping layer is not a free surface. That is, if it is fastened, such as by glue, to a thin plate, this will not allow the interface to move freely along the shear direction. This plate may be called constraining plate and does not have to be thick. It may be a thin aluminum sheet, or even a Nylon sheet, of a fraction of the thickness of the damping layer. If the damping layer is 1.5 mm, the constraining plate layer may be 0.5 mm. Or, if a constraining plate layer is not available, a thick damping layer, say 3 mm, may be a substitute. The additional mass above 1.5 mm may be taken as the constraining plate layer. At the head frame, the bumper guard can serve the purpose of a constraining plate layer very well.
The damping layer, in the present application, may even be formed from a composite plate consisting of a number of plies of different grain orientation to optimize the shear-damping characteristics even further. Therefore, in general, we shall name the damping device as a damping layer assembly consisting of the damping layer which may have multiple plies plus the constraining plate layer at the top. The said plies may be assembled ply by ply, one upon another, to become an integrated composite layer and applied to the frame by glue or other means. Or, the plies may be sprayed on under rigid control for material and thickness options to the surface of the frame of the racket at specific locations of the frame, with the constraining plate layer added at the end, if it is added al all.